Embodiments of the present invention relate to programming an antifuse under low voltage power supply and process limitations for an integrated circuit in either wafer or packaged form.
Referring to FIG. 1A, there is a current-voltage diagram of a semiconductor controlled rectifier (SCR) of the prior art. FIG. 1B is a simplified diagram of the SCR showing the PNPN impurity layers and intervening junctions J1-J3. Here and in the following discussion it should be understood that a semiconductor controlled rectifier may also be called a silicon controlled rectifier or a thyristor as described by S. M. Sze, “Semiconductor Devices Physics and Technology” 148-156 (John Wiley & Sons 1985). In general, a silicon controlled rectifier is a special case of a semiconductor controlled rectifier that is specifically formed on a silicon substrate. The current-voltage diagram shows a reverse blocking region 100 where junctions J1 and J3 are reverse biased, but junction J2 is forward biased. By way of contrast, junctions J1 and J3 are forward biased, but junction J2 is reverse biased in the forward blocking region 102. At switching voltage Vsw 104, the SCR switches from the forward blocking region to a holding voltage (Vh) and holding current (Ih) region 106. In this mode all three junctions J1-J3 are forward biased and the minimum holding voltage across the SCR may be as low as a single diode drop or approximately 0.7 V. In holding region 106, therefore, the SCR functions as a near ideal switch for programming fuses or antifuses.
Antifuses are preferably formed by two conductive terminals separated by an intervening dielectric as disclosed by Cutter et al. in U.S. Pat. No. 6,444,558, and incorporated by reference herein in its entirety. Prior to programming, antifuses typically have a very high resistance on the order of 1e9 ohms. The antifuse is typically programmed by placing a voltage across the intervening dielectric to produce an electric field in excess of 10 MV/cm. This is sufficient to rupture the dielectric, but the antifuse resistance may still remain high and unpredictable. After dielectric rupture, therefore, the antifuse is subjected to a relatively high current of 10-30 mA for a short period of time and often at a lower voltage than required for dielectric rupture. This is often referred to as soaking the antifuse to melt and alloy the conductive material that penetrates the ruptured dielectric. The resulting programmed antifuse may have a stable resistance of less than 250 ohms. The relatively high soaking current, however, requires a circuit with correspondingly large MOS transistors.
Rung has disclosed a circuit to program fuses with an SCR at U.S. Pat. No. 4,605,872 and incorporated herein by reference in its entirety. The circuit disclosed by Rung at FIGS. 1-3, however, provides a relatively high holding voltage due to the spacing between critical regions of the SCR. Furthermore, the circuit of Rung would require a separate fuse latch and is not well suited to parallel programming. These and other problems are resolved by the following embodiments of the present invention as will become apparent in the following discussion.